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CHAPTER 5: STRUCTURE OF POLYMERS

CHAPTER 5: STRUCTURE OF POLYMERS . "The time has come," the Walrus said, "To talk of many things: Of shoes--and ships--and sealing-wax-- Of cabbages--and kings--". Lewis Carroll, Through the Looking Glass (1872). shoes, ship, sealing wax, cabbage, and a king The "many things" listed by the Walrus are actually very similar in chemical composition and STRUCTURE . If we consider only wooden ships, then all of the things on the list are composed of large carbon frameworks called POLYMERS . Since 1872 chemists have identified the common POLYMERS produced by plants and animals, primarily proteins (collagen, keratin) and carbohydrates (cellulose, starch). Chemists have also learned to synthesize new POLYMERS from simple chemicals, creating a vast array of plastics and synthetic fibers. PROPERTIES OF POLYMERS . In CHAPTER 1 it was shown that metals, POLYMERS and ceramics have contrasting physical and chemical properties.

5 - 3 5.3 LEWIS STRUCTURES Rather than writing a sentence for the number of valence electrons on an atom, it can be more useful to draw a picture containing this information.

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Transcription of CHAPTER 5: STRUCTURE OF POLYMERS

1 CHAPTER 5: STRUCTURE OF POLYMERS . "The time has come," the Walrus said, "To talk of many things: Of shoes--and ships--and sealing-wax-- Of cabbages--and kings--". Lewis Carroll, Through the Looking Glass (1872). shoes, ship, sealing wax, cabbage, and a king The "many things" listed by the Walrus are actually very similar in chemical composition and STRUCTURE . If we consider only wooden ships, then all of the things on the list are composed of large carbon frameworks called POLYMERS . Since 1872 chemists have identified the common POLYMERS produced by plants and animals, primarily proteins (collagen, keratin) and carbohydrates (cellulose, starch). Chemists have also learned to synthesize new POLYMERS from simple chemicals, creating a vast array of plastics and synthetic fibers. PROPERTIES OF POLYMERS . In CHAPTER 1 it was shown that metals, POLYMERS and ceramics have contrasting physical and chemical properties.

2 Cotton t-shirts and plastic spoons do not have much in common with bicycle frames or coffee mugs. POLYMERS have low densities; do not reflect or absorb light (they are white or colorless); do not conduct electricity; and are flammable. A primary reason that polymer properties are different is because the chemical compositions of metals, POLYMERS and ceramics are totally different. POLYMERS are composed of non-metallic elements, found at the upper right corner of the periodic table. Carbon is the most common element in POLYMERS . The chemical bonds in POLYMERS are also different than those found in metals and ceramics. 5-1. COVALENT BONDS. Non-metallic elements have a high number of valence electrons (four or more) and prefer to gain electrons, not lose them, in chemical reactions. They often form anions. In a compound of only nonmetals there are no elements willing to become cations, so ionic bonds are not possible.

3 Instead, two nonmetallic atoms can share valence electrons with each other. This type of electron sharing, called covalent bonding, keeps the shared electrons close to both atomic nuclei. One pair of shared electrons makes one covalent bond. A molecule is a group of atoms held together by covalent bonds. This type of bonding contrasts with metallic bonding, in which valence electrons are not associated with a particular nucleus, and move easily throughout a sample. To determine how many covalent bonds can be formed between atoms, first the number of valence electrons must be counted. This can be determined by using a periodic table. The group number matches the number of valence electrons. Example: Carbon is element 6. It is found in Group IV so has four valence electrons. Oxygen is element 8. It is found in Group VI so has six valence electrons. The molecule carbon dioxide has the chemical formula CO2.

4 4 electrons from C + 2(6) electrons from O = 16 valence electrons 5-2. LEWIS STRUCTURES. Rather than writing a sentence for the number of valence electrons on an atom, it can be more useful to draw a picture containing this information. A Lewis STRUCTURE for an atom starts with a chemical symbol, with a dot added for each valence electron. H C O. Since the highest possible number of valence electrons is eight (for noble gases) the dots representing valence electrons are traditionally arranged on four sides of the symbol, with at most two electrons on each side. Experiments have shown that most nonmetallic nuclei are satisfied when they are near eight valence electrons. That is known as the octet rule. Carbon has four valence electrons, and needs to find four more to share. Oxygen has six valence electrons, so it only needs two more. Hydrogen is an exception to the octet rule; its nearest noble gas, helium, has only two electrons.

5 Hydrogen nuclei form molecules with two nearby electrons, a duet rule. To show a covalent bond, two chemical symbols are put near each other with two dots, representing a pair of electrons, between them. For example, a water molecule has one oxygen atom covalently bound to two hydrogen atoms. water O H. H. Nuclei do not have to share all of their valence electrons. Note that two pairs of the oxygen's valence electrons are not shared with any other atoms. Those electrons are called lone pairs or nonbonded pairs and can influence the chemical properties of a molecule. Because dots can be difficult to see, it is common to draw a line segment for each bond (two electrons). The Lewis STRUCTURE of a water molecule then looks like water O. H H. Hydrogen peroxide is a different compond of hydrogen and oxygen, with chemical formula H2O2. Since hydrogen atoms only need a duet of electrons they are found at the outside of the molecule; the two oxygen atoms need to be in the center where they can form more bonds.

6 In this textbook that structural information will be given by underlining the central atom(s) in the chemical formula: H2O2. O O. hydrogen peroxide H H. Sometimes nuclei will need to share more than one pair of electrons to achieve an octet 5-3. with the available valence electrons. One shared pair of electrons is a single bond. Two shared pairs (four electrons) makes a double bond, and three shared pairs (six electrons) makes a triple bond. The more electrons that are shared, the stronger the bond will be. The neighboring elements carbon, nitrogen and oxygen commonly use double and triple bonds. For example, both nitrogen and oxygen are usually found as diatomic gases, N2 and O2. The nitrogen molecule has 2x5 = 10 valence electrons, and oxygen molecule has 2x6 = 12. Nitrogen needs a triple bond to achieve octets for each atom, but a double bond is sufficient for the oxygen molecule.

7 N N O O. The type of covalent bond affects the shape of a molecule. The nuclei move closer together if they share more electrons. This means that a triple bond is shorter than a double bond, which is shorter than a single bond. The bond angles are also very specific in a covalently bond molecule. The shared electrons want to be near the two positively charged nuclei, but try to stay away from negatively charged lone pairs. This STRUCTURE is quite different than that created by metallic bonds, which do not have a particular orientation. Covalent molecules can flex a bit under stress but prefer to "bounce back". to their original positions. Although a Lewis STRUCTURE is a good way to show covalent bonds between atoms, it is not as effective at showing a molecule's three dimensional shape. Chemists use model kits and chemical graphics programs to visualize the positions of atoms in molecules.

8 FUNCTIONAL GROUPS IN POLYMERS . Carbon is the most important element in POLYMERS . Because it starts with only four valence electrons, and wants to share four more, carbon forms a wide variety of covalent bonds. Most importantly, carbon forms strong bonds with itself. Long, strong chains or nets made of thousands of carbon atoms form the backbone of a polymer . C C C C C C C C. C C. C C. carbon backbones 5-4. Polyethylene is the simplest polymer . In addition to the carbon backbone, only hydrogen atoms are used to achieve four covalent bonds per carbon atom. H H H H. polyethylene C C C C. H H H H. Although silicon is in the same group as carbon, it does not form strong bonds with itself. Silicones, long chains of alternating silicon and oxygen atoms, can be synthesized. silicone Si O Si O Si Many different nonmetal atoms could be covalently attached to a polymer backbone. Groups of atoms that contribute something besides C-C and C-H bonds are called functional groups.

9 They affect the chemical and physical properties of a polymer . Examples of Functional Groups O. acid group (-COOH) C H. O. alcohol group (-OH) O H. H. amino group (-NH2) N. H. chloride (-Cl ) fluoride (-F). styrene (-C6H6). 5-5. The primary experimental method used to identify functional groups in POLYMERS is Infrared Spectroscopy (IR). This technique is described in CHAPTER 14. SKELETON STRUCTURES. Simplified or "skeleton" structures can be used to emphasize the functional groups. Carbon-carbon bonds of the framework are represented by line segments. Each vertex is the location of a carbon atom. Most hydrogen atoms and all lone pairs are omitted. This type of diagram deemphasizes the hydrocarbon skeleton; since it is so strongly bonded as to be unreactive, it does not affect the chemical properties of the polymer . Polyethylene is the simplest polymer . Since it has no functional groups, the skeleton STRUCTURE of a polyethylene fragment looks like it does not have any atoms!

10 (Remember that a real polyethylene molecule is more often 100 or 1000 atoms long.). polyethylene It is possible to figure out the missing information. There should be a carbon atom at the end of each line segment; six are needed, connected by five single bonds. C C. C. C C C. Since each carbon atom must have four bonds in a molecule, there must be missing bonds to hydrogen atoms. For the carbon atoms on the ends of the molecule, adding three C-H bonds to each will achieve octets. Two C-H bonds should be added to each of the inner carbons. H H. H H H H. C C C. H H. C C C. H H H H H. H. complete Lewis STRUCTURE for polyethylene fragment three dimensional model of polyethylene 5-6. When functional groups are added to a simplified backbone it is easy to notice the change in STRUCTURE . Polyfluoroethylene, often sold as Teflon, is similar in STRUCTURE to polyethylene except that all the hydrogen is replaced with fluorine.


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